Electrical apparatus

ABSTRACT

The maximum acceptable operating current of an electrode immersed in an electrically conductive liquid is increased by providing at least one shield member in front of the electrode and serving to reduce the current density at the part or parts of the electrode where current density would otherwise be highest.

BACKGROUND OF THE INVENTION

The present invention relates to electrical apparatus of the kind havingan electrode having an area exposed to current flow through a conductivemedium, e.g. a conductive liquid, which may or may not contain solids.Such a conductive medium will be referred to herein as an "electrolyte"for convenience irrespective of the mode of conduction in the liquid andof whether any electrolysis occurs. Such an arrangement is found inelectrical apparatus intended for many different purposes, for instanceohmic heating apparatus, electrolysis apparatus and electricalbatteries.

The performance of such apparatus is often limited by the maximumcurrent density which the electrode can sustain. It is often found thatthe current density is not constant over the whole of the surface areaof the electrode exposed for current flow but tends to increasesubstantially at certain parts of the electrode, typically at the edgesof the electrode. This may be because additional current paths areprovided by a volume of electrolyte situated laterally beyond the edgesof the electrode or may be due to the shape of the electrode. In orderto keep the maximum current density to which any part of the electrodeis subjected down to an acceptable level, the bulk of the surface areaof the electrode has to be operated at a current density which isconsiderably less than would be preferred.

In some ohmic heating cases it may be possible to avoid this problem byincreasing voltage rather than current so that the desired heating powercan be obtained at a permissible current density. However, increasingvoltage may cause problems such as operator safety, or damage by arcing.Unless means for limiting the current are included higher voltages willthemselves lead to higher currents passing in a given system. The use ofcurrent limiting factors may in itself cause problems.

It may be possible to increase electrode size but this may causeproblems. Electrode materials are frequently expensive, e.g.incorporating precious metal coatings and in some forms of apparatussize may be inherently undesirable. For instance, in ohmic heaters usedfor heating foodstuffs, the use of a large electrode area implies theuse of large heating chambers. This in turn makes rapid heating moredifficult. Slow heating is undesirable because of the consequent loss offlavour, vitamins, texture or other factors affecting quality throughlong exposure of foodstuffs to heat.

It would be desirable therefore to provide some means for limiting theincrease of current density at particular areas, e.g. towards the edges,of the electrode in such apparatus.

It has been proposed previously, e.g. in GG-A-526238 and U.S. Pat. No.2,584,654, to vary the proportion of the area of an electrode immersedin an electrolyte which is effective in passing current by positioning amoveable shield member between the electrode and a counter electrode.Movement of the shield member to expose a varying amount of theelectrode to current flow may be used to control the gross current flowthrough the apparatus. The aim of the shield member in such systems isnot to control current density at particular locations on the electrodebut simply to limit the overall current flow so that with the shieldmember in place, the apparatus operates substantially below its maximumcurrent capability.

SUMMARY OF THE INVENTION

The present invention provides electrical apparatus comprising anelectrode having an area exposed for current flow, and a shield memberin a fixed relationship with the electrode, said shield member beingthin relative to the length of the current path in the apparatus andbeing so positioned with respect to the electrode as to reduce currentdensity at the part or parts of the electrode surface which in theabsence of the shield member would experience the highest currentdensity, the shield member thereby serving to increase the currentcarrying capacity of the electrode.

Electrodes in apparatus according to the invention may have a widevariety of shapes including plate-like, cylindrical or spherical. Theareas on the electrodes which would normally experience the maximumcurrent density will depend on the electrode shape and sometimes on thenature of the counter electrode with which the electrode defines acurrent path. The current density maximum may occur at the closestapproach of the electrode to the counter electrode, where current pathsare shortest. Often it will occur at an edge of the electrode whereelectrolyte outside of the direct path between the electrode and itscounter electrode provides additional current paths. Generally, theshield member will be positioned between the electrode and its counterelectrode, overlying that area of the electrode surface which wouldotherwise suffer the greatest current density. By restricting thecurrent density at these areas relative to the rest of the electrodesurface, paradoxically the current carrying capacity of the system canbe increased. The average current density can be raised without themaximum current density exceeding permitted limits.

By the presence of the shield member, apparatus according to theinvention is enabled to operate at higher overall currents thanotherwise. This is in contrast to the effect of movable shields used torestrict current in known devices.

Preferably the shield member extends face to face with and spaced fromthe shielded area of the electrode.

Preferably, the shield member extends substantially beyond the area ofthe electrode to be shielded.

The thickness of the shield member is preferably less than 10% of thesaid current path, e.g. more preferably less than 5%, e.g. from 0.1 to2%. The shield member is preferably so configured as not to cause anysubstantial obstacle to liquid electrolyte flow transverse to thedirection of current flow.

In preferred embodiments, the present invention provides electricalapparatus comprising an electrode, having an area thereof exposed forcurrent flow, a counter electrode, means maintaining the electrode andcounter electrode in a spaced relationship, means for containing anelectrolyte to occupy the space between the electrode and counterelectrode such that the electrolyte extends laterally beyond at leastone edge region of the area of the electrode exposed for current flow,and an electrically insulative shield member fixedly disposed betweenthe electrode and the counter electrode in a spaced, generally face toface relationship with a or the said edge region of the electrode withinsaid means for containing the electrolyte, said shield member serving toobstruct partially the passage of electric current in use between thesaid edge region of the electrode area exposed for current flow and thecounter electrode.

Preferably, the electrode and counter electrode are disposed in a faceto face relationship.

Preferably, the shield member overlaps the said edge region of theelectrode area exposed for current flow, i.e. extends both laterallybeyond said edge region outside the said electrode area and laterallytowards the middle of the electrode.

Preferably, the electrode and counter electrode are disposed in a faceto face relationship.

Preferably, the shield member overlaps the said edge region of theelectrode area exposed for current flow, i.e. extends both laterallybeyond said edge region outside the said electrode area and laterallytowards the middle of the electrode.

Preferably the shield member overlaps the edge region of the electrodearea exposed for current flow by a distance which is from 10 to 300% ofthe spacing between the electrode and the counter electrode.

More preferably, said overlap is from 10 to 100%, more preferably 20 to50% of said spacing, for instance about 30%.

The shield member preferably extends outwardly beyond the electrode areaexposed for current flow by a distance at least as great as the spacingbetween the electrode and the counter electrode, more preferably by atleast twice said spacing, for instance by at least four times saidspacing.

The electrode may be of any shape and may present any number of edges orany shape of edge. Shield members may preferably be provided overlappingany edge of the electrode bordered by electrolyte.

The apparatus may be provided with a pair of said shield members, eachdisposed between the electrode and the counter electrode in a spaced,face to face relationship with the electrode and each extending over arespective one of two oppositely disposed edge regions of the electrodearea exposed for current flow and each serving to obstruct partially thepassage of electric current in use between the respective said edgeregion and the current electrode.

The apparatus may include a further said shield member disposed betweenthe electrode and the counter electrode in a spaced face to facerelationship with the electrode and extending over a third edge regionof the electrode area exposed for current flow, which third edge regionconnects between the two said oppositely disposed edge regions, saidfurther shield member serving to obstruct partially the passage ofelectric current in use between the said third edge region and thecounter electrode.

The first, second and third mentioned shield members may be integralwith one another.

The apparatus may include a second further said shield member disposedbetween the electrode and the counter electrode in a spaced face to facerelationship with the electrode and extending over a fourth edge regionof the electrode area exposed for current flow, which fourth edge regionconnects between the two said oppositely disposed edge regions and isopposite to said third edge region, said second further shield memberserving to obstruct partially the passage of electric current in usebetween the fourth said edge region and the counter electrode.

The fourth shield member may be integral with the first, second andthird mentioned shield members.

The shield member therefore may comprise a plate having an aperturetherein to overlie the central region of the electrode.

In some cases it may be preferred that the or each shield member ispositioned substantially midway between the electrode and the counterelectrode.

However, the or each said shield member may be positioned nearer to theelectrode than to the counter electrode and, optionally, for the or eachsaid shield member associated with the electrode, a corresponding shieldmember may be provided for the counter electrode, the arrangement ofshield members being substantially symmetrical about the mid pointbetween the electrode and the counter electrode. This would generally bepreferred when the inter electrode spacing is large in relation to theelectrode dimensions.

Where the electrode is planar, preferably the or each shield member is asubstantially planar member.

An alternative preferred electrode/counter electrode configuration isone where a first of the electrode and the counter electrode is a rod ortube having an external cylindrical electrode surface and the other ofsaid electrode and counter electrode is tubular and concentricallyoverlies the first presenting an internal cylindrical electrode surface.An anular space is thereby provided to be occupied by electrolyte. Atthe end of the electrode and counter electrode a cylindrical shieldmember of a diameter intermediate that of the electrode and that of thecounter electrode and of relatively small wall thickness may beprovided, overlying the end region of the electrode. A number of suchelectrode and counter electrode pairs may be provided spacedlongitudinally from one another. The electrode or counter electrodewhich is outermost of the two may preferably form a tubular containmentvessel for an electrolyte or may line the wall of such a vessel.

Such an arrangement provides a substantially unobstructed flow path forliquid electrolyte through the annular space between the electrodes.

Generally speaking, whilst it is desirable that the shield member berelatively thin, the degree of thinness of the shield member in thebetween electrodes direction will not be critical. However, the shieldmember may have to sustain substantially all of the voltage applied tothe cell and must therefore have sufficient dielectric strength to standthe voltage in question.

For many forms of apparatus, the electrode and counter electrodepreferably have substantially planar working surfaces which aresubstantially parallel to one another and to the or each shield member.

However, particularly in cathodic protection apparatus or in groundingelectrodes, the electrode may be a bar-shaped electrode and the shieldmember may take the form of a collar substantially surrounding the endregion of the electrode area exposed for current flow. The shield membermay overlap the end region of the electrode area exposed for currentflow. Similar shield members may be provided around joints in theelectrode, which otherwise would tend to be areas of high currentdensity.

The shield member may be provided with a closure portion which may beintegral with the remainder of the shield member and which closes saidcollar over the end region of the electrode area. For instance, theshield member may take the form of a cup having a base and side wallspositioned over the end of a bar shaped electrode which is received withclearance within the side wall.

Preferably, a pair of said shield members will be provided, one at eachend of the electrode in those cases where the electrode is whollycontained in the electrolyte.

The effect of the shield member should be to make more uniform thecurrent density at the electrode and in particular to reduce the currentdensity at the edge which is shielded.

Depending on the nature of the apparatus, the maximum current density atthe edge may be restricted to no greater than twice the average currentdensity when the apparatus is in use, in some cases to no greater than1.5 times, and in some cases to as low as 0.65 times the average.

The apparatus may include means for applying a potential differenceacross said electrode and counter electrode, which may be means forapplying an alternating potential difference or a constant potentialdifference.

The apparatus may comprise a plurality of sets of said electrodes andcounter electrodes.

The apparatus may be such that the electrodes and counter electrodesshare an electrolyte in common.

The means for containing the electrolyte may be a tubular member havinga plurality of said electrodes disposed in a longitudinally spacedrelationship along an internal surface thereof and a correspondingplurality of said counter electrodes disposed in a similarlongitudinally spaced relationship there along, e.g. along an internalsurface thereof or extending along a central axis thereof, facing saidelectrodes.

The apparatus may be ohmic heating apparatus.

Alternatively, the apparatus may be for electrolysis, electro-winning,electrostripping, electroplating, electroforming, electrochlorination,or may be an electrical primary or secondary battery, an electricaldisplay device, for instance an electrochromic device, a liquid crystaldevice, or an electroluminescent device or may be a cathodic protectioninstallation or a liquid pumping device.

The invention includes a sacrificial anode for use in cathodicprotection which anode is bar shaped and has an electrically insulativeshield member in the form of a collar substantially surrounding each endof the electrode. Each shield member may be provided with a closureportion closing each said collar across the respective end of theelectrode.

In a further aspect the invention includes electrical apparatuscomprising an electrode, having an area thereof exposed for currentflow, a counter electrode, spaced from the electrode, an electrolyteoccupying the space between the electrode and counter electrode suchthat the electrolyte extends laterally beyond at least one edge regionof the area of the electrode exposed for current flow, and anelectrically insulative shield member fixedly disposed between theelectrode and the counter electrode within the electrolyte in a spaced,generally face to face relationship with the said edge region of theelectrode and serving to obstruct partially the passage of electriccurrent in use between the said edge region of the electrode areaexposed for current flow and the counter electrode.

Such apparatus may in particular be a cathodic protection installation.The counter electrode may be an object to be protected and theelectrolyte may be a liquid, such as water, with which both theelectrode and said object are in contact.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to preferred embodimentsillustrated in the accompanying drawings in which:

FIG. 1 is a schematic longitudinal cross-section (not to scale) throughan ohmic heating apparatus according to the invention;

FIG. 2 is a graph illustrating the effect of the invention in limitingcurrent density at the edge of an electrode;

FIG. 3 is a graph illustrating the variation in uniformity of currentdensity with the placing of a shield member;

FIG. 4 is a side view of an electrode for use in cathodic protectionaccording to the invention;

FIG. 5 is an isopotential plot for apparatus generally of the kind shownin FIG. 1; and

FIG. 6 is an isopotential plot similar to that of FIG. 5 but fordifferent operating conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a longitudinal cross-section, the heating chamber of ohmicheating apparatus 10 having a first pair of opposed insulating sidewalls 11 into which are set at intervals on one face thereof threeelectrodes 12 and on an opposite face thereof three counter electrodes13. A second pair of opposed insulating side walls 11a serves to spaceside walls 11 to define the said heating chamber. The heating chamber isof square cross-section and the electrode and counter electrodes areidentical to one another.

In use, the interior of the chamber is filled with a fluid to be heatedsuch as water, milk or a flowable foodstuff material which serves as anelectrolyte to complete a current path between the electrodes 12 and thecounter electrodes 13. The nature of the heating chamber is such thatthe electrolyte extends laterally beyond the transversely running edgesof the electrodes into areas 14.

To prevent excessive current density in the edge portions of theelectrodes and counter electrodes in use, the apparatus is provided withshield members 15 and 16. These are of insulating material of sufficientdielectric strength to resist the voltage to be applied to the apparatusin use. The shield members 15 are positioned at the ends of the heatingchamber overlap the edges of the electrodes 12 and the counterelectrodes 13 so as to extend over the electrode and counter electrodeconcerned by a distance t which is approximately 30% of the spacingbetween the electrode and the counter electrode. The shield member 15extends away from the electrode and counter electrode concerned by adistance which is approximately the same as the spacing between theelectrode and the counter electrode.

Between successive pairs of electrodes there are positioned shieldmembers 16 which overlap with each of two electrodes 12 by a distance t.The shield members 16 extend beyond the edge of each electrode 12 awayfrom the electrode by a distance which is over twice the spacing betweenthe electrode and the counter electrode. The shield members serve toobstruct current flow from the edge regions 12a of the electrodes 12.

The thickness of the shield members 15, 16 seen edge on in FIG. 1 isexagerated. In practice the shield members in this embodiment take theform of thin plates of insulating material having a thickness of about1% of the inter-electrode spacing.

It will be appreciated that in a system such as the one described inwhich there is more than one pair of electrode and counter electrode,additional current paths are presented between successive electrodeswhen alternating voltage is applied to the electrodes and counterelectrodes of the apparatus. Thus, current can flow not only between theelectrode 12 and its corresponding counter electrode 13 but also betweensuccessive electrodes 12 if there is any voltage difference between themas there will be if different phases of a multi-phase supply are appliedto the electrodes. The shield members 16 help to prevent excessivecurrent density at the edges of the electrodes 12 and counterelectrodes13.

FIG. 2 is a graph schematically indicating the current density asdetermined by a computer simulation from the centre of an electrode 12to its edge in the presence of a shield member overhanging the edge by adistance t. The graph is a simplification in that the computersimulation used considers only a two dimensional slice through theelectrode. In the graph shown in FIG. 2, the distance t is taken to be 3cm, 20 cm and the separation between each electrode 12 and thecorresponding counter-electrode 13 is taken to be 10 cm.

The maximum current density is about 1.2 times the current density atthe electrode centre. In the absence of the shield member the ratiowould be substantially greater.

FIG. 3 shows two plots of the ratio between the current density at theedge (J edge) and the current density at the centre (J centre) againstthe ratio of the overlap distance t and the inter-electrode spacing L.The upper plot is for an inter-electrode 12 spacing of 5 cm and anelectrode to counter electrode spacing L of 15 cm. The lower plot is foran inter-electrode 12 spacing of 40 cm and an electrode to counterelectrode spacing of 15 cm.

It can be seen that the evening out of the current density across theelectrode depends mainly upon the degree of overlap t between the shieldmember and the electrode and very little upon the spacing between theelectrodes 12.

FIG. 4 shows a sacrificial anode according to the invention comprising abar shaped electrode 20 equipped at each end with a cup shaped shieldmember 21 having a base 22 and a circumambient wall extending from thebase toward the centre of the electrode and spaced from the electrodesurface. In the use of such a sacrificial anode, the apparatus to beprotected will constitute the counter electrode of electrical apparatusaccording to the invention and the conductive medium in which theapparatus to be protected is situated will constitute the electrolyte.

The effect of the use of the shield members described above in apparatusof the kind schematically shown in FIG. 1 is further illustrated inFIGS. 5 and 6 which are plots of isopotentials produced by computersimulation. FIG. 5 indicates that in normal use there ill be virtuallyno current flow between electrodes on the same side of the pipe meansand that the ratio between the maximum current density at any electrodeand the average current density will be about 1.5. This will mean thatthe average current density may be much higher than would be the case inthe absence of the shields without the edge current density becomingexcessive. The heating effect is greatest where the isopotentials areclosest and it can be seen that the heating effect will therefore begreatest along the central axis of the heater. Since the flowablematerial is likely to flow fastest along the central axis, this currentdistribution is likely to result in more uniform heating. Furthermore,the positioning of the shield members along the central axis will itselftend to reduce the flow rate along the centre of the pipe means, onceagain helping to produce uniform heating.

FIG. 6 illustrates the condition which may momentarily arise whencurrent is switched off at the centre electrodes. The plot indicatesthat there is not excessive current flowing between the upstream anddownstream electrodes respectively and the central pair of electrodesand also indicates that the maximum current density at the upstream anddownstream pairs of electrodes is even under these conditions notexcessive in proportion to the average current density.

Both plots indicate that there will be virtually no current flow to theneutral electrodes at the end of the pipe means.

Many modifications and variations of the invention as described aboveare possible within the general scope of the invention. The shieldmember characteristic of this invention may be incorporated into ohmicheating apparatus of the kind described in our British PatentSpecification No. 8,716,673 and may be used in conjunction withelectrodes of the kind described in our British Patent Application No.8,725,866.

A number of other conformations of electrodes and shields will readilyoccur to those skilled in the art.

The flat electrodes and shields principally illustrated herein may bereplaced by tubes or conical sections, such as would arise if one viewedthe arrangement of electrodes and counter-electrodes shown in FIG. 1 asbeing shown in half section with a centre line lying parallel to orobliquely to the axis of the arrangement presently shown.

We have described above an arrangement in which a rod electrode isshielded at each end by a tubular shield arrangement extending beyondthe rod. A number of rods and shields may be combined in a twodimensional parallel array (like the nails in a bed of nails) withalternate ones being of opposite polarity to produce an electro-chemicaleffect.

In an electro-galvanizing plant in which a coil of steel is passedaround a roller in a plating bath and acts as an electrode, shieldmembers might be provided for each edge of the web in the form of curvedplates overlying the edges of the web but spaced therefrom to allow freecirculation of liquid.

Another alternate conformation would involve two flat metal ringelectrodes, each mounted on the face of an insulating base, set in aface to face parallel or converging position. A conductive liquid mayflow radially inwards or outwards to or from a centre tube running in onthe axis of one of the electrodes. The shield members may be two ringsof insulator, one overlapping the outer edge of the electrodes and onethe inner. For heating the liquid, the interelectrode distance may bevaried in the direction of liquid flow to obtain a desired heatingpattern by varying the resistence time and current density to takeaccount of changes in conductivity with temperature.

We claim:
 1. Electrical apparatus comprising:an electrode and a counterelectrode, each having an area exposed for current flow; flow path meansfor defining a flow path of a liquid through said electrical apparatusand over said electrode and counter electrode; and shield means forincreasing the current carrying capacity of the electrode and counterelectrode by reducing current density at parts of the electrode surfaceand counter electrode surface which in the absence of the shield meanswould experience the highest current density; said shield meanscomprising at least one shield member in a fixed relationship with theelectrode and counter electrode, said at least one shield member beingthin relative to the length of the current path in the electricalapparatus.
 2. Electrical apparatus as claimed in claim 1, wherein saidat least one shield member extends face to face with and is spaced froman are of the electrode to be shielded.
 3. Electrical apparatus asclaimed in claim 1, wherein said at least one shield member is spacedfrom and extends substantially beyond an area of the electrode to beshielded.
 4. Electrical apparatus as claimed in claim 1, wherein said atleast one shield member is fixedly disposed between the electrode andcounter electrode in a spaced, generally face to face relationship withan edge region of the electrode area exposed for current flow. 5.Electrical apparatus as claimed in claim 4, wherein said at least oneshield member overlaps said edge region of the electrode area exposedfor current flow.
 6. Electrical apparatus as claimed in claim 5, whereinsaid at least one shield member overlaps said edge region of theelectrode area exposed for current flow by a distance which is from 20%to 50% of the spacing between the electrode and counter electrode. 7.Electrical apparatus as claimed in claim 5, wherein said at least oneshield member extends outwardly beyond the electrode area exposed forcurrent flow by a distance at least as great as the spacing between theelectrode and counter electrode.
 8. Electrical apparatus as claim 4,wherein said flow path means comprises a tubular member having aplurality of said electrodes disposed in a longitudinally spacedrelationship along an internal surface of said tubular member, and acorresponding plurality of said counter electrodes facing saidelectrodes disposed in a longitudinally spaced relationship along theinternal surface of said tubular member.
 9. Electrical apparatus asclaimed in claim 1, wherein the flow path of said liquid through saidelectrical apparatus is transverse to the direction of current flow. 10.Electrical apparatus as in claim 1, wherein the shield member allowsliquid to flow over said electrode and counter electrode.
 11. Electricalapparatus comprising:an electrode and a counter electrode, each havingan area exposed for current flow; flow path means for defining a flowpat of a liquid through said electrical apparatus and over saidelectrode and counter electrode; said flow path means comprising atubular member having a plurality of said electrodes disposed in alongitudinally spaced relationship along an internal surface of saidtubular member, and a corresponding plurality of said counterelectrodes, facing said electrodes, also disposed in a longitudinallyspaced relationship along the internal surface of said tubular member;shield means for increasing the current carrying capacity of theelectrode and counter electrode by reducing current density at parts ofthe electrode surface and counter electrode surface which in the absenceof the shield means would experience the highest current density; saidshield means comprising a plurality of shield members fixedly disposedbetween each electrode and its respective counter electrode in a space,generally face to face relationship with an edge region of the electrodearea exposed for current flow; said shield member being thin relative tothe length of a current path in the electrical apparatus.
 12. Electricalapparatus comprising,an electrode and counter electrode, having an areaexposed for current flow; spacer means for maintaining the electrode andcounter electrode in a spaced relationship; containing means forcontaining a liquid to occupy the space between the electrode andcounter electrode such that the liquid extends laterally beyond edgeregions of the areas of the electrode and counter electrode exposed forcurrent flow; passing means for passing said liquid through saidcontaining means to flow over said electrode and counter electrode; andan electrically insulated shield member fixedly disposed between theelectrode and the counter electrode in a spaced, generally face to facerelationship with said edge regions of the areas of the electrode andcounter electrode exposed for current areas of the electrode and counterelectrode exposed for current flow; said electrically insulated shieldmember partially obstructing the passage of electric current at saidedge regions of the electrode and counter electrode exposed for currentflow.